1. Field of the Invention
This invention relates to oil industry wellbore perforating, especially perforating by means of tubing conveyed perforating apparatus run in combination with production tubing that permits one-trip well perforating and completing.
2. Description of the Prior Art
Tubing conveyed perforating (TCP) of oil wells has been known for decades, and is now gaining in popularity. Its advantages over wireline conveyed perforating include rig time savings, higher shot density, greater gun length, and the ability to perforate "underbalanced" (i.e. with pressure differential into the borehole) so that perforations are cleansed through formation fluid backflow.
Perforating guns are sometimes run below production tubing to enable the well to be perforated and completed (i.e. put into production) in one operation. Once the guns have fired, they are dropped by means of a gun release mechanism to the bottom of the well, thereby leaving an unobstructed path for the flow of formation fluids up through the tubing.
Various techniques have been used to fire the guns in permanent completion TCP. Typical among these are drop-bar or "go-devil" systems, electrical firing systems and early hydraulic systems, such as the firing in response to sudden exposure through the opening of a valve to the difference between a stored reference pressure and cushion pressure in the tubing. Problems have been encountered, however, in applying the latest "controlled" hydraulic firing techniques, such as described in Applicant's copending U.S. patent applications Ser. Nos. 369,209 and 476,074, to permanent completion TCP.
Such "controlled" hydraulic firing systems offer inherent safety advantages over other systems and are thus attractive candidates for use in permanent completion perforating. For example, in the method set forth in U.S. Ser. No. 369,209 reference, firing occurs in response to the controlled development from the surface of a predetermined pressure differential between pressure in the well annulus (the annular space above the packer between the tubing string and the walls of the borehole) and pressure in the tubing communicating with the "rathole" (the region below the packer in which the guns are positioned). Utilization of such "controlled" hydraulic systems for permanent completion requires solution to the problems of routing annulus or rathole pressure through the packer to the firing head while maintaining an unobstructed flow path and good annulus-to-tubing sealing when the guns (and firing head) are subsequently dropped.